The invention relates to a method for MR (=magnetic resonance) imaging and spatially resolved MR spectroscopy using an MR tomograph to reduce artifacts occurring due to the motion of an object to be represented, wherein the object position is determined quasi-continuously during the runtime of the MR acquisition, which includes one or more partial acquisitions, and wherein motion correction is performed, which comprises dynamic adaptation of the frequency and phase settings of the RF (=radio frequency) system of the tomograph and of the orientation and amplitudes of the gradients during the runtime of the MR acquisition according to the current object position.
Such a method is known from US 2009/0209846 A1 (=reference [1]).
MR tomography, also known as magnetic resonance imaging (=MRI), MR imaging or magnetic resonance tomography (=MRT) is a non-invasive method that makes it possible to spatially resolve the inner structure of objects in three dimensions and to represent them. It is based on the energy behavior of atomic nuclei in a magnetic field, which permits excitation of its nuclear spins by suitable radio-frequency pulses, followed by analysis of the reaction. MR imaging is mainly used in medicine to provide a view into the interior of a human body.
The signal emitted from the atomic nuclei of the object being examined as a reaction to excitation with radio-frequency pulses is read using suitable receiver coils. The spatial encoding required to be able to allocate the measurement signal to a position within the object to be represented is achieved with additional, spatially variable magnetic fields Bz(x,y,z) that are superposed on the static main magnetic field B0, causing atomic nuclei at different positions to exhibit different Larmor frequencies. Conventionally, magnetic fields are used that exhibit the most linear possible variation in intensity in the relevant spatial direction, termed constant or linear magnetic field gradients. The customary gradient systems produce three orthogonal gradients in the x-, y-, and z-directions in this way, but local gradient systems are also used in spatial encoding. 1-, 2-, or 3-dimensional spatial encoding is performed by varying the magnetic field gradient in all three spatial directions according to known principles, for example, Fourier encoding, filtered back-projection, or another known method [11].
To generate a signal that can be used for MR, spatially variable magnetic fields and RF pulses are superposed on a stationary magnetic field. Error-free, in particular, artifact-free mapping is only possible if the measurement object to be represented is completely motionless for the entire representation process.
Image artifacts and signal disturbances that occur due to motion during the representation process limit the image quality that can be achieved. In everyday clinical practice, patient movements can result in unusable acquisitions, which can result in lost time because acquisitions have to be repeated, or even incorrect diagnoses.
The possibility of detecting and correcting movements that occur is therefore an essential factor in ensuring the quality of the data obtained by MR. In particular, in everyday clinical practice, avoidance of motion artifacts increases the efficiency of the measurement procedure and considerably reduces the costs. Such a procedure for realtime motion correction during the MR acquisition is known from [1] cited above.
To avoid motion artifacts in MR acquisitions, three basic approaches are known:    1.) In so-called gating, the acquisition is interrupted as soon as the measurement object is no longer in the desired position and resumed as soon as it is in the desired position again. Gating of respiratory motion is used especially frequently for acquisitions of the abdominal and thoracic regions of patients. This method has the disadvantage that the duration of an MR acquisition is considerably increased.[2]    2.) In retrospective correction, the MR acquisition is performed without change even when an object movement occurs; it does not alter the temporal progression of the acquisition. However, retrospective correction is only possible for a limited number of image artifacts. Retrospective methods are no use for movements that result in inconsistent image objects or signal loss. [3]    3.) Prospective motion correction enables reduction of the artifacts caused by patient motion by adapting the volume to be represented to the current position of the measurement object. For this purpose, the position of the measurement object to be represented is sensed by a measurement system or measurement method throughout the MR acquisition. If movement of the measurement object is observed, dynamic adaptations of the frequency and the phase settings of the radio-frequency system (RF system) of the tomograph and of the orientation and amplitudes of the gradients are performed.
One possible measurement method acquires the position data using MR navigator sequences. These navigator sequences require a short interruption in the sequence and can therefore only be used for certain acquisitions and permit only a few corrections per second. [4, 5] One promising approach is installation of a measurement system (e.g. an optical measurement system) inside or outside the tomograph. In this way, it is possible to obtain information about the object position concurrently with MR acquisition. [1, 6-8]
An acquisition by means of MR tomography generally consists of individual partial processes comprising signal generation and/or signal acquisition with a typical duration >4 ms. Before such a partial acquisition starts, all the parameters (gradients, RF pulses, carrier frequencies and phases) required for the partial acquisition are calculated.
Two basic approaches for prospective motion correction are described:    1.) For correction using a software interface, calculation of individual partial acquisitions is adapted to the changed position of the measurement object and the relevant parameters are calculated based on this position. The calculated sequence parameters are then transmitted digitally to a signal generator.[1, 6-10]    2.) In motion correction using a hardware interface, these digital signals of calculated partial acquisitions are adapted to the changed position of the measurement object. During short idle phases (<2 ms) between individual partial acquisitions, the digital signals of the following partial acquisition are adapted to the changed object position.[1, 9]
[6] describes a method for motion correction for arbitrary acquisitions. Before slice excitation, adaptation to the object position is performed. As an additional option, after the signal read-out period, the object position is determined again and if it exceeds certain limits of motion, the partial acquisition is performed again. Movements during the remaining temporal progression of the individual partial acquisitions are not corrected.
[7] describes a configuration of a system for motion correction. The motion correction is performed at the beginning of each partial acquisition. Motion correction during the temporal progression of the individual partial acquisitions is not described.
[1] describes a method for motion correction for arbitrary acquisitions. For the actual application of the motion correction, two possibilities are suggested (using a software interface and using a hardware interface). In the first case with a software interface, motion correction is performed at the beginning of each sequence-specific partial acquisition. Motion correction during the temporal progression of the individual partial acquisitions (in particular, uninterrupted motion correction of continuous gradients) is not described. In the second case using a hardware interface, the motion correction is performed in the short idle periods between 2 partial acquisitions. Motion correction during the temporal progression of the individual partial acquisitions is likewise not described.
[8] describes a configuration of a system for motion correction. At the beginning of each partial acquisition, one motion correction is performed, but not during the temporal progression of the individual partial acquisitions.
[10] presents a method for motion correction using the method described in [7]. The object position is only adapted before each slice excitation.
[9] presents a method for motion correction during an acquisition for diffusion-weighted imaging (DWI) using the method described in [1]. For this purpose, motion corrections are used before each slice excitation and before each signal read-out period. Movements that occur between slice excitation and signal read-out period are only detected and the affected partial acquisitions are rejected and repeated.
[5] describes a method that compensates for object rotation during diffusion weighting in DWI by a combination of navigator sequences and additionally run correction gradients. With this correction variant, only artifacts caused by rotations of the measurement object are corrected during the applied gradients. This correction also requires interruption of acquisition to run the navigator sequence and to run the correction gradients. Motion that occurs during the temporal progression of a partial acquisition (depending on the acquisition method, in the range 4 to 1000 ms) can result in serious artifacts and signal loss. None of the methods presented so far describes a way of correcting these—the only way is to repeat the partial acquisition(s) affected, which prolongs the duration of the MR acquisition accordingly.
In contrast thereto, it is the object of this invention to still further reduce the artifacts caused by movements of the object to be represented in an MR imaging or spectroscopy method described above.